docs: def handle_unset_citation(self, line: str, position: int, tokens: ParseResults) -> ParseResults:
          """Unset the citation, or raise an exception if it is not set.
  
          :raises: MissingAnnotationKeyWarning
          """
          if not self.citation:
              raise MissingAnnotationKeyWarning(self.get_line_number(), line, position, BEL_KEYWORD_CITATION)
  
          self.clear_citation()
          return tokens